1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy electrode in an alkaline storage battery and to a method for manufacturing such an electrode.
2. Description of the Prior Art
Alkaline storage batteries, such as Ni-Cd batteries, and lead acid batteries, are well-known. Currently, lighter and larger capacity batteries with high energy density are needed. Therefore, metal hydride storage batteries, which utilize a hydrogen-absorbing alloy to reversibly absorb and desorb hydrogen as a negative electrode, would appear to be advantageous.
In a hydrogen-absorbing alloy electrode for an alkaline storage battery, hydrogen-absorbing alloy granules are mixed with a powdery binder to form the electrode. The surface of the electrode is covered with a thin film composed of the binder with low oxygen permeability.
One of an important characteristics of the metal hydride storage battery is rapid charging characteristics. When charging is terminated in a short time using high rate current, that is, when the battery is rapidly charged, the utility of the positive electrode decreases. As a result, oxygen gas is generated at the positive electrode. The oxygen is consumed by reacting with hydrogen in the hydrogen-absorbing alloy. Since the surface of the negative electrode is covered with a thin film composed of the binder, having a low permeability of the oxygen gas, the negative electrode is inferior to oxygen gas absorption characteristics. Because the oxygen gas consumption reaction does not proceed rapidly enough, the internal pressure of a battery equipped with such a negative electrode increases.
On the other hand, on manufacturing the hydrogen-absorbing alloy, each component of the alloy is measured and mixed in a fixed ratio, melted in an arc furnace with an inert argon atmosphere and then cooled to obtain a hydrogen-absorbing alloy bulk. To complete a negative electrode, the hydrogen-absorbing alloys are mechanically pulverized into granules. The granules are then kneaded into a paste with powdered polytetrafluorethylene (PTFE), which acts as a binder. The paste is coated onto a conductive substrate, as a current collector, comprising a punched metal plate.